This application is the U.S. national-phase application of PCT International Application No. PCT/GB98/02718.
This invention concerns a method of refining platinum group metals. More especially it concerns a method of interseparation of platinum group metals from a feed solution. The present invention has particular application in the production of high purity platinum group metals.
It has been proposed to use gel chromatography to separate platinum group metals from one another. A problem which exists with the current technology in that metal impurities, such as base metals and other impurity elements, must first be separated from the feed solution before the solution is subjected to chromatographic separation as these impurities interfere with the separation. This has posed a significant obstacle to the use of gel chromatography for the separation of platinum group metals on an industrial scale.
Prior proposals to use chromatographic separation for platinum group metals include U.S. Pat. No. 4,855,143 (Schmuckler) and EP 756013 (Matthey Rustenburg Refiners (Pty) Ltd). The latter represents a major step forward in the design of practical separation methods, but there remains a need for an improved method which is capable of the treatment of actual feedstocks from a PGM refinery, which are contaminated with a variety of base metals and amphoteric elements, and which is capable of yielding very high purity platinum and palladium.
The present invention provides a method of interseparation of platinum group metals from a feedstock solution in hydrochloric acid which solution may contain said metals in admixture with base metals and amphoteric elements, comprising the steps of:
(a) adjusting the acidity of the feedstock solution to be in the range from 5.5 to 6.5M HCl, preferably 5.7 to 6M HCl;
(b) adjusting the redox potential of the feedstock solution to at least about 500 mV;
(c) passing the acidity- and redox-adjusted solution through a chromatographic medium causing the base metals, other impurity elements and the platinum group metals to interact with the chromatographic medium, and
(d) eluting at least platinum and palladium in separate fractions from the chromatographic medium using an eluent.
The feedstock solution in a real refinery will contain a large number of elements, as their salts and compounds. Depending upon the source of the feedstock, the platinum group metals will be present in different proportions, desirably these include platinum and palladium principally, with smaller amounts of Rh, Ru, Ir and Os. Other metals which are expected to be present include principally Au, Ni, Fe and Cu, with lesser amounts of Ag, As, Bi, Pb, Te etc, as well as Sb, Al. Desirably, the feedstock contains Pt in concentrations of up to about 100 g/litre. Such feedstocks may exhibit crystallisation, eg of Ni, if the concentration of Pt is significantly above 100 g/litre. In general, we prefer to treat larger volumes of lower concentration than smaller volumes of higher concentration, in the present invention. It is to be noted that this is generally the reverse to conventional chromatography.
Suitable chromatographic media include polymethacrylate and polysaccharide or polystyrene gel beads, which are commercially available under the trade marks Sephadex G10, MacroPrep CM, Source 15ETH and ToyoPearl. Preferred media for use in the present invention are ToyoPearl HW40-EC, ToyoPearl HW40-F or especially ToyoPearl HW40-C.
The preferred eluent is approximately 6M hydrochloric acid. Preferably, a large volume of eluent is used compared to the volume of feedstock; the acidity of the eluent may be optimised to individual feedstocks in order to achieve desired selectivities.
In the method of the invention, adjustment of the redox potential of the feedstock solution is desirably carried out by oxidation of the solution or a combination of oxidation and reduction steps. For example, oxidation may be carried out by an oxygen or air sparge, eg at elevated temperature for about 1 hour. Conventional means, eg a pH/redox meter with a platinum electrode together with silver/silver chloride reference electrode, may be used to measure the redox potential of the solution. Preferably, the redox potential is in the range 550 to 600 mV.
Suitable feedstock solution may be readily available as a refinery stream. If required, however, a feedstock solution may be prepared by starting from a solid PGM concentrate. Preferably, such a solid concentrate is thermally treated. This may be carried out by, for example, roasting at about 750xc2x0 C. for about 1 hour in air or oxygen. An alternative thermal treatment involves roasting as before, followed by a nitrogen flush for 15 minutes, followed by reduction at about 750xc2x0 C. for about 1 hour, using hydrogen or a suitable hydrogen-containing gas, eg cracker gas.
Desirably, such a thermally treated solid feed is subjected to a base metal leach before being dissolved. Base metals may be partially leached from the solid feed, primarily to reduce iron content to below 5 g/litre in the leach liquor. If the analysis shows an iron content  greater than 5 g/litre in the leach liquor, the leach is desirably repeated once or possibly twice until this is achieved. Preferably, leaching is carried out using hydrochloric acid, suitably 10N HCl, and suitably using a solid to liquid ratio of about 1:3.5 (kg/litre), at about 85xc2x0 C. for about 1 hour. After completion, the solids were allowed to settle for half an hour before the supernatant leach liquor was analysed for iron.
The leached feed may then be dissolved. A suitable dissolution procedure is to suspend the solids in 6M HCl, suitably at a concentration of 1:8 (kg/litre) solids to liquid ratio, followed by agitation at elevated temperature, eg at 70xc2x0 C., and with chlorine addition, eg using a chlorine sparge, for sufficient time to dissolve the PGMs present. This has been found to be about 13 hours in experimental work. After switching off the flow of chlorine, the resulting solution is boiled to remove excess chlorine. The residue after such a dissolution step is desirably filtered off.
It is preferred to remove substantially all dissolved gold from the feedstock solution before chromatographic separation. This may be done conveniently by a solvent extraction, suitably using MiBK (methyl isobutyl ketone), although other methods may be used if desired. This is also effective to remove the majority of Se, Te, Sn, Sb and Fe. The resulting liquor is evaporated to low volume to remove any MiBK in the solution then back diluted to its original volume using 6M HCl.
It is believed that certain selenium-metal complexes may exist in solution, and should desirably be removed therefrom, if they are present, before chromatographic separation. This may be carried out conveniently by passing the solution through a bed or column of activated carbon.
Although the method of the invention permits the separation of at least Pt and Pd from refinery streams which are contaminated with base metals and amphoteric elements, it should be understood that other feedstocks containing Pt, Pd and possibly Os, which are not significantly contaminated with such metals and elements, may readily be interseparated to yield high purity platinum group metals.
The method of the invention may be carried out using a variety of apparatus types, for example using a batch column, a simulated moving bed chromatograph, a continuous annular chromatograph or a xe2x80x9cGatling gunxe2x80x9d type of chromatograph, and variations on these. Such apparatus and variations thereof are described in the art. The present invention has been successfully carried out both on laboratory-scale 10 mm columns and scaled up to 300 mm columns.
The invention is described in more detail in the following passages of the specification which refer to the accompanying drawing and chromatograms. The specific features and characteristics of the invention described in the passages which follow should merely be seen as illustrative of the invention and not limiting on the overall scope of the invention.